The Systems Alliance is an independent supranational government representing the interests of humanity as a whole. The Alliance is responsible for the governance and defense of all extra-solar colonies and stations.
The Alliance grew out of the various nation space programs as a matter of practicality. Sol's planets had been explored and exploited through piecemeal nation efforts. The expense of colonizing entire new solar systems could not be met by any one country. With humans knowing that alien contact was inevitable, there was enough political will to jointly fund an international effort.
Still, the Alliance was often disregarded by those on Earth until the First Contact War. While the national governments dithered and bickered over who should lead the effort to liberate Shanxi, the Alliance fleet struck decisively. Post-War public approval gave the Alliance the credibility to establish its own Parliament and become the galactic face of humanity.